For Nebraska there are two sets of functional classification maps to reference. One set has the full counties, but doesn't show the areas within some of the counties most significant cities. The other set focuses just on the primary cities within those counties.
Please use both resources depending on what areas you're editing in. NOTE, some gravel roads are considered collector types and based on the new road type standards being in consideration for "Primary Street" status despite the fact they are unpaved roads. This may ultimately need a larger discussion if it becomes problematic for routing, but there are those gravel roads that are wider, and better maintained than others that have earned the "collector" status and are meant for more traffic regardless of their unpaved nature.